|Linnaeus , 1758|
The Atlantic salmon ( Salmo salar ) belongs to the genus of salmon and lives mostly in the Atlantic Ocean . In late autumn, however, the salmon migrate far up the rivers of Europe and North America to spawn in the upper reaches . The fish can be up to 1.5 meters long and need gravel bottoms to spawn. At the end of these “spawning migrations”, the females lay their eggs and the males fertilize them. Since the migration and the spawning act are very strenuous for the animals and they often do not eat any food on the way, most of the salmon die of exhaustion or illnesses caused by it before they reach the open sea again.
The fry hatch after one to five months, depending on the water temperature. By around a year old, they are strong enough to migrate into the oceans . After a few years, they too return to their place of birth to spawn there.
The Atlantic salmon was named Fish of the Year in Germany in 1992, 2000 and 2019 and in Switzerland in 2015 .
After two years in the sea, Atlantic salmon are on average 71–76 cm long and 3.6–5.4 kg in weight, although individual specimens can also grow larger.
The coloring of young Atlantic salmon does not resemble the adult coloring. While they live in fresh water , they have blue and red spots. In the ripening phase they take on a silvery-blue shimmer. The adult animals are best recognized by the black dots mostly above the lateral line organ . However, the caudal fin is usually not spotted. During the mating season , males take on a light green or red color. The salmon has a spindle-shaped body and well-developed teeth.
In addition to the species names, there are also colloquial names for the different age forms for the Atlantic salmon. "Parr" is the name given to salmon that are no more than two years old and differ from the "Smolts" by their "Parr" markings (dark vertical bars on the flanks). In older records, such young salmon are also called "Sälmling". Young salmon (but also sea trout ) on their first trip into the sea are called “smolt” . At this stage the fish are mostly silver in color. "Grilse" is the name given to the salmon when they first come back to the upper reaches of the rivers to spawn from the sea. The salmon has usually spent a year and a half to two years in the sea. At first it comes as “blank salmon” until it becomes colored salmon and the males finally develop the spawning hook (“hooked salmon”).
distribution and habitat
The distribution of the Atlantic salmon depends on the water temperature. Due to global warming , certain southern populations are expected to disappear soon in Spain and other warm countries. Before human intervention, rivers in Europe and on the east coast of North America were the natural breeding grounds for Atlantic salmon. When North America was colonized by Europeans, eggs were brought by train to the west coast and introduced into the rivers there. There have also been other efforts to introduce Atlantic salmon into new settlement areas, such as B. New Zealand. Because there are no suitable ocean currents in New Zealand , most of these attempts failed. In New Zealand there is at least a local population of Atlantic salmon, but they never go into salt water ("landlocked").
Young salmon spend one to four years in their home river. When they are big enough (approx. 15 cm), they undergo a physiological change in which they change the protective color with large gray dots, which is adapted to rivers, to a protective color with shimmering sides adapted to the sea. They are also subject to certain internal changes, ie they adapt to the changed osmosis during the transition from fresh water to salt water . In the end, the "parr", the young salmon, end their changes by swimming with the current instead of against the current. When this behavior change occurs, they are no longer called "parr" but "smolt". When the "smolts" reach the sea, they follow the currents on the sea surface and feed on plankton or fry of other fish species, such as fish. B. Herring . While they are in the sea, they can use the lateral line organ to orient themselves to the earth's magnetic field .
After a year of good growth, the surface currents carry them back to their home river. It is a misconception that salmon swim thousands of kilometers in the ocean; but they “surf” with the help of the ocean currents. When you reach your home flow, you may recognize it through olfactory perception ; only 5% of Atlantic salmon migrate up the wrong river. Thus, the habitat of Atlantic salmon consists of the river in which they were born and the currents of the ocean surface that are connected to this river in a cycle.
In the 20th century, wild salmon populations disappeared in many rivers due to overfishing and habitat changes. By the year 2000, the number of Atlantic salmon had fallen to a critically low level.
Life cycle and migration
Most Atlantic salmon follow a spawning site-related migration pattern in which most of their feeding and growth takes place in salt water. For spawning, however, adult salmon return to native freshwater rivers, where the young hatch from the eggs and go through various growth phases.
Atlantic salmon do not need salt water. There are examples of populations that live exclusively in freshwater (“landlocked”). In North America, these salmon strains are known as "ouananiche".
The freshwater phase of Atlantic salmon varies between one and eight years, depending on the location of the river.
The first stage is the "alevin" stage, in which the fish stay in the hatchery and feed on the remaining nutrients from the yolk sac. During this phase, their gills develop and they become active hunters. Next comes the “fry” stage, when the fish grow and leave the hatchery to forage. During this time they go to regions with a higher concentration of prey. The final freshwater stage is their development to "parr", where they prepare for the migration into the Atlantic Ocean.
Salt water phase
When "parrs" develop into "smolts", they begin their migration into the Atlantic Ocean, which mostly takes place between March and June. The hike enables acclimatization to the changing salinity . When they are ready, the young “smolts” leave the home river. For this they prefer low tide .
When they leave their home rivers, they go through a period of rapid growth in the ocean for one to four years. Typically, Atlantic salmon migrate from their home rivers to a region in the western Greenland continental plate . During this time they are especially endangered by predators such as B. Human, seal , Greenland shark , rays , cod and halibut . Certain dolphins have been seen playing with dead salmon, but it is still unclear whether they will eat them too. Once they weigh more than 250 g, they are no longer easy prey for birds and other fish, although gray seals and common seals continue to hunt them. The survival rate is now between 14 and 53%.
Then the Atlantic salmon are big enough and ready to return to their original freshwater habitat. You switch to the "grilse" stage. After the migration to their home waters, the salmon stop eating completely before spawning. It is still largely unknown to what extent the smell - the exact chemical composition of the river - plays an important role in the return of the salmon to its homeland.
Young salmon begin to feed themselves within a few days. As soon as the yolk sac is used up, they start hunting. The fry start with very small invertebrates and as they get bigger they occasionally eat smaller fish. During this time they hunt on the bottom and in the current. Some eat salmon eggs. The most common foods are caddis flies , black flies , mayflies, and stone flies .
"Fry" and "parr" are said to be territorial, but there is no conclusive evidence that they are defending their territories. Even if they show aggressiveness towards one another, their social hierarchy is still unclear. Often they move on as a swarm when they leave the estuary of their home river.
Adult Atlantic salmon are considered to be much more aggressive than other salmon and are more likely to attack other fish. A problem is when salmon have become an invasive threat and attack native salmon, e.g. B. King salmon and silver salmon .
The situation in Germany
Salmo salar was at home in Germany well into the 20th century. He spawned in the tributaries of the Rhine , u. a. the victory . At that time, the Rhine was considered the most important and largest salmon river in Europe. Place names such as Salmtal as well as coats of arms or inns still remind us of the great importance of the Rhine salmon at that time.
The salmon population declined sharply due to human intervention in the ecosystem of rivers. The reasons were overfishing or the remodeling of the rivers. The discharge of industrial sewage also worsened the living conditions for numerous fish species. All salmon disappeared from the Rhine and its tributaries by the middle of the 20th century. In 1950 the last salmon disappeared from the victory.
In 1987 the International Commission for the Protection of the Rhine (ICPR) decided on the " Salmon 2000 Project ", which was supposed to make salmon at home in German rivers again. It was successful in that the water quality could be greatly improved (but this does not mean good) and the first reconstruction measures were initiated, e.g. B. the beginning with distances from migration obstacles. The aim of the follow-up project "Salmon 2020" is salmon populations that can sustain themselves in the Rhine system. Today, the salmon can be found again sporadically in the Rhine and its tributaries as well as in the Elbe. However, it will be decades before there are self-reproducing salmon populations again. At the moment, hundreds of thousands of young salmon are released every year, of which only a fraction naturally returns.
For freshwater habitats it has been granted legal protection in accordance with the Habitats Directive .
Genetically modified salmon
The first genetically modified animal intended for human consumption could be a transgenic Atlantic salmon. The GM salmon with the brand name AquAdvantage have a gene for a growth hormone from another salmon species (king salmon) and another gene from the fish species Zoarces americanus, which is adapted to cold sea regions . With these two genes, GM salmon produce more growth hormones. Instead of three years, it is ready for slaughter after 16 to 18 months. The application was made in the US in 1995 and the safety tests required by the FDA have been passed (GM salmon are just as safe as other salmon, according to the FDA). Among other things, it had to be ensured that the genetic modifications remain stable and do not have any negative effects on animal health. All animals are also female and sterile and should be kept in closed tanks so that undesired outcrossing is (theoretically) impossible.
The US Food and Drug Administration (FDA) approved a genetically modified animal as a food for the first time on November 19, 2015 . The edible fish with the name AquAdvantage Salmon is bred by the US company AquaBounty Technologies . The transgenic salmon are fully grown within 16 to 18 months. Without a genetic change, this takes 30 months for Atlantic salmon. The US company operates breeding stations in Canada and Panama.
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